If you slowly pour more water into the bowl, does the water level rise
and stay at a level you'd consider acceptable?
If it won't there's not much you can do about it, you're stuck with the
design of that toilet bowl and it's trap geometry.
If it does stay "higher" then you may have a blocked ventpipe system
which leads to "sucking" water out of the bowl. Learn how to
check/correct that or consult a professional.
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