So, it's happening with random Markers within the same file (in other words, it's not all of the Markers at the same time?).
I've had a similar situation with elevation markers in AC11 changing their pens, seemingly at random, and I've had to reset the Marker Object (in other words, set the Marker head to a different one, and then back again) to the default settings. I believe I've only seen this with the new Interior Elevation Markers.
The "Built-In" Markers are typically hard-wired, so if it's a problem with the GDL code, you wouldn't be able to fix it, anyway. Depending on the number of Markers you are talking about, you may want to replace them with AC11 Markers.
MacBook Pro Apple M2 Max, 96 GB of RAM
AC27 US (5003) on Mac OS Ventura 13.6.2
Started on AC4.0 in 91/92/93; full-time user since AC8.1 in 2004