5 years later and this is still really annoying, assuming this is the same issue in Udon Graph where these value types get reset to 0 when you reload the graph. LayerMask does this too and will get reset to Nothing.

The only way around this issue seems to be to use public variables as constants (private variables will also get reset) or convert from another type, like Int

added conversions for char field in basic nodes, still a to-do for char array fields and char default value fields in the Variables window.