Unity games are compiled for Windows (x86_64 architecture). Your Android phone uses ARM64. The plugin must translate every single instruction. This extra work pushes your CPU to 100% usage constantly, generating heat.
[1] Lee, S., & Kim, B. (2018). A study on the development of a social media plugin for Unity game engine. Journal of Game and Entertainment Technology, 13(2), 1-12. unity plugin for joiplay hot