Sometimes, a lot of the time, when a player joins the Realms server, my skin will change to theirs. 

I.E

Player A is an Indiana Jones Skin
Player B is a Megaman Skin

When Player B logs in, he has the correct skin and also sees the correct skin on Player A.

However, from Player As perspective, Player As skin has changed to Megaman. 

In order to fix, Player A has to go to Profile and reactive their skin.

Now, when there are a few people on I often get comments as "Why is my skin suddenly different?". I explain what happened and they go through the steps to correct it on their end.

I feel this happens to custom uploaded skins and not to skins you utilize from the Bedrock Skin service. I.E skin creation tool or micro transactions.