I had this issue because I had a dependency on Microsoft.Extensions.Configuration.Json in project B that targeted netstandard. Microsoft.Extensions.Configuration.Json requires System.Text.Json when starting .NETStandard, but not dotnetcore.
My problem came when in project A that targeted dotnetcore3.1 referenced project B. At runtime, AWS Lambda was still expecting System.Text.Json to be there.
To resolve the issue, I ended up switching project B to target dotnetcore3.1 as well, even though it is a pure library and not something executable.
Not sure this answers your question directly as I don’t fully understand your situation, but a possible solution for this scenario. It was a little difficult finding many other resources on this issue but I probably just don’t know what to look for.