I believe what is happening here is:
- MIDI items are always beat-based, regardless of project or track timebase setting. As you note, the workaround is to set the individual MIDI media item to "ignore project tempo, use XXX."
- When changing time signatures, beat-based positions of media items are preserved. When inserting the earlier 10/8 time signature, the MIDI media items remain at the same number of quarter notes from the start of the project -- not the same number of seconds, or bars.
- When inserting time signatures, later time signature change markers remain at the same bar location.
The problem you are seeing is the difference between how media items are handled and how time signature markers are handled, when earlier time signature changes are inserted. A time signature change at bar 35 will remain at bar 35 regardless of time signatures changing earlier in the project. A beat-based media item at bar 35 will end up in a different bar if earlier time signature changes change the total number of beats in a bar.
You could make a reasonable argument that a beat-based item at bar 35 should remain at bar 35 even if the total number of beats occurring before bar 35 changes. I'm not saying one behavior or the other is correct or wrong, I'm just explaining the reasons why the current behavior exists.
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