If the version a1.2.0 is selected in the launcher, the launcher will download the version a1.2.0_02 instead. This is an issue with the launcher, not the client. This is also a universal issue, as even the wiki page for the Halloween Update highlights this problem.
I will reiterate that this is a launcher issue, not a client issue. Either the files should be replaced or version a1.2.0 should be removed from the launcher altogether.
MCL-14707
Downloading a1.2.0 through the launcher downloads a1.2.0_02 instead
Resolved
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Hello Cameron! I'm sorry that this is an issue. We're tracking this issue as MCL-1421, but it unfortunately will not be fixed (unless the launcher developers decide to reopen the report). 😞