Note that there has been discovered a caching regression on dictionary null values in Umbraco 13 and 16 due to HybridCache.
It has been addressed in Umbraco 13 with: Caching: Fixes regression of the caching of null representations for missing dictionary items (closes #20336 for 13) by AndyButland · Pull Request #20344 · umbraco/Umbraco-CMS · GitHub
And will land soon in Umbraco 16: Caching: Fixes regression of the caching of null representations for missing dictionary items (closes #20336 for 16) by AndyButland · Pull Request #20349 · umbraco/Umbraco-CMS · GitHub
I cannot say for sure if this is your problem, but wanted to add it here just in case.