ZDoom

No, because this thing is a real mess to fix, and only gets into play if you pick up such a thing right after spawning.
To make it clear: The feature as implemented is not fixable, it will have to be reimplemented from scratch. If you want it fixed, how about thinking up a solution yourself? Oh, of course that solution should not break anything, too!

Yeah because there's no such thing as multiplayer, right?

I'll be honest here: I really don't care. Fixing this is a rather complicated affair because the two invulnerability items get merged and that's not easy to undo. It's really not worth the effort for a completely cosmetic issue with a near-zero likelihood of appearing outside of deliberate test cases.

zdoom -iwad doom2 -warp 01 -nomonsters -deathmatch +sv_respawnprotect 1 +sv_cheats 1

If you remember [url=http://forum.zdoom.org/viewtopic.php?f=2&t=46152]this bug report[/url], now there's something which could be nice to fix:

[code]
zdoom -iwad doom2 -warp 01 -nomonsters -deathmatch +sv_respawnprotect 1 +sv_cheats 1
[/code]

active the chasecam, add a bot, kill yourself, then when you respawn, 'summon invulnerabilitysphere' and pick it up when the respawn protection is still in effect. You will notice that the protection effect will disappear only if the invulnerability expires, which is after 30 seconds instead of 3 seconds.