Thank you for your feedback. I checked it on the last version of Resting (v0.12.0 - Firefox 66) and it seems OK.
My test:
1.I created context1 and define url variable with value 'www.google.com'
2.I created context2 and define url variable with value 'www.yahoo.com'
3.I created a request putting {url} in the url field.
4.I bound request with context1 (using context tab) and the execution used www.google.com as expected
5.I bound request with context2 (using context tab) and the execution used www.yahoo.com as expected
6.I bound request with context1 again (using context tab) and the execution used www.google.com as expected

Please if your problem is not resolved, feel free to post here ( or on the issue tracker of Resting https://github.com/mirkoperillo/resting/issues)
the steps to reproduce exactly your problem, I'll give it max priority