[Bug] Opening GUI multiple times causes items to be cleared

[notkazu]

Plugin version

1.5.14

Server software and version

Paper 1.21.4, Build #185

Steps to reproduce

https://www.youtube.com/watch?v=aIhL9I52ZKs

Expected behaviour

.

What is actually happening?

.

Relevant log output

.

Any additional comments?

This issue does not occur as often when the server is not under load, but it is still possible, as shown in the video.