When alternating between multiple Garmin watches during the day, both with TrueUp enabled, number of steps is not being kept in sync between them. In this case it's a fenix 6 Pro (primary tracker) and a fenix 5 Plus. I've noticed this has been the case for months now - can't say for sure it has ever worked with the fenix 6 Pro tbh.
nb. use case is me wearing the 6 Pro nearly all the time, apart from when I'm just about to head out for a long activity and realise I haven't charged it lately, so switch to my old 5 Plus while the 6 Pro gets plugged it.
Back in the day I used to regularly alternate between a Vivosmart 4 and a fenx 3 (then a fenix 5 Plus) and this worked fine: I'd wear the Vivosmart 4 throughout the day and rack up steps, then I'd switch to the fenix when doing an activity and after its first connection/sync with my phone the fenix would show the number of steps already racked up for that day from the Vivosmart, similarly once I was done and switched back with Vivosmart would update with the steps collected from the fenix.
Also of note is that other functions of TrueUp (activities, recovery, load etc) do sync just fine.