I found that the cassette save process is aborted shortly after the cassette interrupt has been armed; the same happens for loading. The load/save routines are quite similar to the 99/4A, some addresses adjusted. Looks like one of the more difficult bugs to find, if the bug is in MAME. I took a note for later resolution.
Any TI-99/8 user can check this (preferably Ciro, whose ROMs are used in MAME) by loading some BASIC program, not too short, from floppy and then trying SAVE CS1. This does not require a real cassette recorder. If the save process stops after about 11 seconds (which is too early), the issue is in the ROM. I am not sure whether the timing is 100% precise, so you could retry with programs of different lengths.
Edited by mizapf, Wed Jan 16, 2019 4:02 PM.