Primary time format switches to secondary time format by itself a lot.
1 - Exemple to reproduce the behavior : While you have Timecode as primary and Bars&beats as secondary, use Set Timecode At Cursor, primary will switch to Bars&beats, being exactly the same as secondary.
2 - Just work a few hours, it’ll primary and secondary will change to the same time format (always bars&beat here) without you noticing it
I first noted it since version 6.5.1 in my book, but it is an older bug than that.