The fuel heater wouldn’t ever be downstream of the fuel HP pump; it's the LP fuel that waxes up. (See notes)

However, in this heat exchanger you will have high pressure fuel and relatively low pressure oil, so any internal leak will transfer fuel to the oil system, raising the indicated oil level.

I have noticed the Notes say "the oil in the FOHE is always at a higher pressure than the fuel".  This is correct when the heat exchanger is in its proper place upstream of the HP pump.