This is as intended. Tags intentionally don't get special treatment and are
revision controlled just like other files. Marking resolved.

can we resolve this ?

you shouldn't care about the head the tag ends.
even if the tag isn't in .hgtags, localrepo.tags does the right thing (it looks
for the tags in all heads).

If I have multiple heads, say at changes 5 and 10, and 5 is the current tip, and
I tag 10:

hg tag thingy 10

Then the tag ends up being part of .hgtags in tip, rather than in 10.

This is explicable in terms of how hg tagging works, but its a bit
counterintuitive; I would expect tagging another branch would add the tag to
that branch's .hgtags (and that 10 would be the parent of that change).

Perhaps the simple thing is to make hg tag warn if you're adding a tag referring
to one head to another head.