Cannot add friend

Unable to add one particular friend only. Gamertag Kizpuppy. Kizpuppy is using Nintendo switch and has Nintendo switch online account too.

Both myself and Kizpuppy have all settings to Allow on Xbox Privacy and online settings in respective accounts.

Both are unblocked. Both online.

Whenever press Kizpuppy Add Friend Button, It flashes added and then reverts to add friend. Cannot play together multiplayer on any created worlds, Even try to invite friend that way, click profile, add friend button does not stay changed. This is also happens when trying from both accounts when trying to add respective friend ie Kizpuppy trys to add friend from my gamertag.

We used to be friended but did a block then unblock multiple times in quick succession as a joke. Now since then cannot re-add each other even though shows we are unblocked