A few possibilities.
LNB skew not optimised for horizontal transponders.
Transponder power is not the same for all transponders, perhaps H pol are lower powered compared to V?
The % you are quoting mean very little as you haven't said what that % relates to... signal strength or signal quality?
Given V works on other sats suggests your LNB isn't faulty.
You really need accurate signal analysis to see what exactly is happening. Eg: accurate BER/MER readings.
As you said, the issue only started when 5G commenced with a TX site near you, suggests the issue is 5G interference, which is very common nowadays.
There are some commercial LNB's which offer better performance and have different frequency filters, but they are expensive and you need a separate one each for H & V... and they still may not solve your issue.
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