This is not a bug but a feature
according to https://minecraft.fandom.com/wiki/Rail
(Wither skeleton can't cross rails tracks.
This happens both in single player and online.)
Environment
Java: 1.8.0_51 64 bits
Windows 10 version 1903
Vanila MC
MC-170319
Not a bug (Wither skeleton can't cross rails)
Resolved
Duplicate
0
Indeed working as intended, see MC-33359.