Pressing "=" in input cell inadvertently converts to free-form input

When working inside some Mathematica 12 notebooks (downloaded from the web), when the “=” key is pressed in an Input cell (e.g. when trying to type in “myFunc[r_]:=”), as soon as the “=” key is pressed, the entry changes into the orange free-form input form. What setting in Preferences or Options Inspector is causing this behavior, and how can I disable this?
This does not happen in a normal, blank Mathematica notebook on my machine. Thanks.

Hi Suha,

This is a bug in the initial release of 12.1.1, see this post for details. Install the latest version of 12.1.1 to resolve the issue.

Thank you so much for the helpful prompt reply, Rohit. Didn’t imagine something this would actually be a (well-known) bug!