Saving as .docx is a “convenience service”. DOCX and ODF format have not the same specifications. There is an important common intersection between them regarding basic formatting features but, as soon as the requested feature(s) become more elaborate, you meet divergence. Therefore, formatting directives must be translated into another “dialect”. Unfortunately there is no one-to-one correspondence between the primitives. This means the description for the formatting/layout/feature must be approximated. When you reopen the document, this approximation group is again interpreted and you don’t end up with the initial description. The more editing sessions you have, the bigger the differences between what you expect and what you get.
If you really need (must?) send a .docx, convert your document only once at end of the full creation/tuning/optimisation process. There is still approximation but only once. If your recipient returns corrections, don’t work on the returned .docx to avoid corruption. Instead, identify the changes (“Track Changes” feature can help) and add them manually into your .odt original.
In addition, I remind you that M$ Word claims it can read and create .odt (though Micro$oft, as usual, has implemented a non-conforming ODF variant!). So, you can put the blame on your recipients by sending them your .odt and let them assume the divergences.