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.