On 4 May 2019, at 4:22, Leo Brewin wrote:

I'be had a recurring problem with one file. For some reason when TextMate
reads the film from disk it somehow corrupts the tail of the file […]
If I delete ~/Library/Caches/com.macromates.TextMate/BundlesIndex.binary
before launching TextMate then the file is not corrupted.

Any suggestions would be most appreciated,

That it is not mangled when you delete the bundle index indicates that it is a bundle item responsible for the mangling (when there is no index, bundles are indexed asynchronously after launch, so any bundle import hooks would not be used for files loaded before indexing has completed).

To troubleshoot this, first open Bundles → Select Bundle Item (⌃⌘T).

Use the gear menu to switch to “Semantic Class” and now enter “import” in the filter string.

There are 5 default hooks for importing files (binary property lists, compiled AppleScript, etc.)

If you see more, then that might be the problem.

If this does not find anything, try removing all custom bundles, steps detailed here: https://github.com/textmate/textmate/wiki/Reverting-To-Defaults