The problem of merging git branches to overwrite the code?

merge the code of another branch on the main branch. The code of one class in the main branch is more than that in the other branch, but there is no hint in merge. After merge, it is found that there is no more code in the class of the main branch. I don"t understand why git did this.

Git
Jun.13,2022
MySQL Query : SELECT * FROM `codeshelper`.`v9_news` WHERE status=99 AND catid='6' ORDER BY rand() LIMIT 5
MySQL Error : Disk full (/tmp/#sql-temptable-64f5-1b3a9d8-e779.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
MySQL Errno : 1021
Message : Disk full (/tmp/#sql-temptable-64f5-1b3a9d8-e779.MAI); waiting for someone to free some space... (errno: 28 "No space left on device")
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