2.0TDi power loss after long engine braking

The circumstances of this are not likely to repeat but I want to get to the bottom of what the problem might have been to make any repairs to prevent future problems.

My engine has ran like clockwork for the last 23K miles of my ownership and has done 102K miles, never any hint of sticking turbo vanes like my previous Ocatavias.

I did a 500 mile autoroute journey yesterday towing a very large very overloaded and unbraked trailer, after 400ish miles driving between 50-60mph so everything nicely warmed through I did a long slow descent towards Brive, above 60mph on overrun there is slight swaying so I changed down to 5th to slow the vehicle, the descent steepened and approached 60mph again so downchange and engine braking in 5th gear, then again in 4th gear, in total it was several kms at about 2500 rpm on a closed throttle engine braking.

Then nearing the bottom I got back on the gas to get up to 60mph to gain momentum for the uphill stretch and there was barely any engine power, like a partial loss of boost or the turbo not spooling up, it felt like the initial stages of running out of fuel which manifests on uphill climbs, at also felt and sounded a bit wooly is the best word I can find.

The power very gradually came back, its not like it cleared, it was just like boost slowly building up again but over several minutes, its hard to judge if you have full power when towing that sort of weight as there is no real acceleration, I pulled off at the next aire to check that I didn't have a shredded tyre and when I accelerated back onto the autoroute it had full power and has had since.

So what anomoly could have been caused by engine braking for so long?

Cooling of the pre DPF 02 sensor? Inplausible 02 reading for so long?

Cooling of the turbo vanes? Whilst there was no combustion it was acting as an air compressor so would have been producing heat.

A higher vacuum maintained for a long time, is there perhaps a valve regulator valve that might have opened & then not closed?

Does the wastegate have a vacuum actuator?

No warning lights came up, I will scan for fault codes soon.

All and any suggestions greatly appreciated.