Yes, as it says near the beginning of plugins:help:export_gedcom_file:export_gedcom_file|> Export Gedcom File Plugin Help & Advice:
The philosophy is that ƒh holds the master database, and the exports are secondary copies for other products, perhaps to obtain online hints in Ancestry, FindMyPast, MyHeritage, etc, or to carry on a tablet when visiting record offices, churches, relatives, etc. Thus it is not critical that every tiny detail migrates from ƒh to the secondary product database. Migrating every detail is often not possible anyway, because of the various dialects of GEDCOM in use. Migrating the data back from the other product into ƒh is unlikely to succeed.
That last sentence is most relevant.
In this case,
MyHeritage does NOT support
Place records, so when you export from
FH they are converted into synthetic
Source records with the
Ω prefix. If you check in
Family Tree Builder > View > Sources you will find them.
That is what is being imported and merged back into
FH.
You can exclude them from being exported from the Plugin on its
Extra Options tab
Place Record 0 @P%d+@: setting as mentioned in plugins:help:export_gedcom_file:std_standard_gedcom_5.5#place_records_plac|> Place Records (_PLAC).
Or in
FH after import, you can easily select all the synthetic
Source records with the
Ω prefix and delete them as explained in how_to:delete_a_large_number_of_records|> Delete Any Number of Records.
But you should also review the data imported from
MyHeritage in some depth before merging as advised in glossary:merge_compare_files#merge_compare_procedure|> Merge/Compare Procedure in order to remove any
MyHeritage custom
GEDCOM data.