Texts in FB commands won't print after >15 labels

Hi everybody
this problem has me stumped, maybe you can help?

We are starting single label prints in rapid succession, which works just fine until the 17th label or so, then the printer refuses to print text with FB commands.

The ZPL does not differ from the previous 16 correctly printed labels.  The testprinter is local usb.

This happens on the GK420t and the ZT410 (from around the 20th label onward, according to the customer)

If I reprint the faulty labels on their own (not in rapid succession) they print just fine.

I've played around a bit and found out that the FB commands that follow a maxicode are interpreted correctly.

So I've tested the following
- Changed IPS from 5 to 4: no effect. As before, only the FB texts AFTER the maxicode are printed
- Removed the maxicode definition entirely: no FB texts are printed at all after the 16th label
- Moved the maxicode definition to the top of the ZPL code: all FB print correctly in all 21 labels

Which is nice, but not at all trustworthy because I don't know why this happens, or if this might resurface in a higher label number.

Anybody any ideas what I'm doing wrong here?

Thanks for reading!

GK420t
Firmware: V61.17.17Z
Media type: gap/notch
Print speed: 5 IPS
Thermal trans
usb comm

Correctly printed ZPL
^XA
^MUm
^LS0^LT0^PON
^LH1,1
^CI28
^FO0.125,40.5^GB27.5,0,0.25^FS
^FO27.5,40.5^GB0,28.75,0.25^FS
^FO27.5,46.5^GB69.15,0,0.25^FS
^FO0.125,69^GB96.75,0,0.25^FS
^FO0.125,78.7^GB96.75,0,0.25^FS
^FO0.125,108^GB96.75,0,0.25^FS
^FO1.875,1.25^A0N,2.5^FB400,7^FDABSENDERZEILE 1\&0000000000\&ABSENDERZEILE 2\&ALLEE 12\&28201 NIRGENDWO^FS
^FO46.875,1.25^A0N,5^FR^FD1.0 KG^FS
^FT95,5,1^A0N,5^FR^FB300,1,,R^FD16 OF 21^FS
^FO66.25,7.125^A0N,2.5^FB400,7^FDSHP#: 9589 98HY KVR\&SHP WT: 20.0 KG\&SHP DWT: \&DATE: 23 MAY 2018^FS
^FO14.25,18.375^A0N,3.75^FR^FDSHIP TO:^FS
^FO28.25,18.375^A0N,3.75^FB3000,5^FDADRESSE 1\&4444444444\&ADRESSZEILE2\&STRASSE 13^FS
^FO28.25,37.375^A0N,5^FR^FB800,2^FD8027 IRGENDWO^FS
^FO0.25,42.375^CVN^BD3^FH_^FD0662768027  [)>_1E01_1D961Z50002756_1DUPSN_1D958998_1D143_1D_1D16/21_1D1_1DN_1D_1DIRGENDWO_1D_1E_04^FS
^FO41.25,47.25^A0N,6.875^FR^FDDEU 098 9-00^FS
^FO33.9,54.5^MUd^BY3^BCN,103,N,N,,A^FD4208027^MUm^FS
^FO1.875,70^A0N,5^FR^FDUPS EXPRESS^FS
^FO1.875,75^A0N,3.75^FDTRACKING #: 1Z 958 998 66 5000 2756^FS
^FT96,77.5,1^A0N,10^FR^FD1^FS
^FO6.5,81^MUd^BY4^BCN,203,N,N,,A^FD1Z9589986650002756^MUm^FS
^FO1.875,108.75^A0N,3.75^FB800,8^FDBILLING: P/P\&COD\&REF#1: UPSPLDTEST3^FS
^FT96,113.5,1^A0N,6.25^FR^FB300,1,,R^FDEDI-COD^FS
^FT96,118,1^A0N,1.875^FB250,2,,R^FDxxxxxxxxx xxxxxxxx xxxxx\&xxx xxxxxxxx xx xx^FS
^FO85,145^A0N,1.875^FD93.5A 10/2017^FS
^MUd
^XZ

All FB texts above the maxicode are not printed
^XA
^MUm
^LS0^LT0^PON
^LH1,1
^CI28
^FO0.125,40.5^GB27.5,0,0.25^FS
^FO27.5,40.5^GB0,28.75,0.25^FS
^FO27.5,46.5^GB69.15,0,0.25^FS
^FO0.125,69^GB96.75,0,0.25^FS
^FO0.125,78.7^GB96.75,0,0.25^FS
^FO0.125,108^GB96.75,0,0.25^FS
^FO1.875,1.25^A0N,2.5^FB400,7^FDABSENDERZEILE 1\&0000000000\&ABSENDERZEILE 2\&ALLEE 12\&28201 NIRGENDWO^FS
^FO46.875,1.25^A0N,5^FR^FD1.0 KG^FS
^FT95,5,1^A0N,5^FR^FB300,1,,R^FD17 OF 21^FS
^FO66.25,7.125^A0N,2.5^FB400,7^FDSHP#: 9589 98HY KVR\&SHP WT: 20.0 KG\&SHP DWT: \&DATE: 23 MAY 2018^FS
^FO14.25,18.375^A0N,3.75^FR^FDSHIP TO:^FS
^FO28.25,18.375^A0N,3.75^FB3000,5^FDADRESSE 1\&4444444444\&ADRESSZEILE2\&STRASSE 13^FS
^FO28.25,37.375^A0N,5^FR^FB800,2^FD8027 IRGENDWO^FS
^FO0.25,42.375^CVN^BD3^FH_^FD0662768027  [)>_1E01_1D961Z50002765_1DUPSN_1D958998_1D143_1D_1D17/21_1D1_1DN_1D_1DIRGENDWO_1D_1E_04^FS
^FO41.25,47.25^A0N,6.875^FR^FDDEU 098 9-00^FS
^FO33.9,54.5^MUd^BY3^BCN,103,N,N,,A^FD4208027^MUm^FS
^FO1.875,70^A0N,5^FR^FDUPS EXPRESS^FS
^FO1.875,75^A0N,3.75^FDTRACKING #: 1Z 958 998 66 5000 2765^FS
^FT96,77.5,1^A0N,10^FR^FD1^FS
^FO6.5,81^MUd^BY4^BCN,203,N,N,,A^FD1Z9589986650002765^MUm^FS
^FO1.875,108.75^A0N,3.75^FB800,8^FDBILLING: P/P\&COD\&REF#1: UPSPLDTEST3^FS
^FT96,113.5,1^A0N,6.25^FR^FB300,1,,R^FDEDI-COD^FS
^FT96,118,1^A0N,1.875^FB250,2,,R^FDxxxxxxxxx xxxxxxxx xxxxx\&xxx xxxxxxxx xx xx^FS
^FO85,145^A0N,1.875^FD93.5A 10/2017^FS
^MUd
^XZ