The tab key is completely glitched in the latest preview. Before I could press tab to autocomplete without any issues, but now it always types a weird character (ō) alongside autocompleting the command. This is not a duplicate of MCPE-17104 or MCPE-158584, this issue only started in 1.20.30.
Since MCPE-17104 mentions the exact same character as you get when typing the tab key, even though the result is slightly different, we are still using that report to track this.
If you have additional information, please add a comment to that report.