In the folder Garmin\Segments there are files in format "[Name]_segment.fit" where [Name] is a name of segment which is limited to 18 symbols.
The problem with a limit of 18 symbols.
For instance you have 2 segments: "Downhill From Point A" (21 symbol) and "Downhill From Point B" (21 symbol). The names of both segments will be truncated to "Downhill From Poin" (18 symbols). The file names of both segments will be "Downhill_From_Poin_segment.fit". In this case the only one segment from two can be synced and stored on watches.
I think that this is a quite critical issue and the amount of symbols should be increased or as alternative - changed the segment file name conversion to use prefix/suffix with segment unique identifier.
In case you are experiencing the same issue please contact with garmin support here https://support.garmin.com/en-US/ql/?focus=topics and tell about this issue.