What should I do about a tensioned/trued wheel that lost significant spoke tension after first use?

It's normal. You can corroborate by letting the air out of your tire and measuring then. The whole thing is one of the reasons tension balance is important; outliers that still seem to have a decent amount of tension on them can be brought to slack by this effect.

I would work on getting the tension balance tighter. Low spoke count wheels are not forgiving of it. DT Spoke Freeze works well as an after-the-fact "prep" if you want something on there without having to undo the wheel. It is true to its name, making re-adjustment later difficult, so you want things pretty dialed first.

Yes, it’s normal for spoke tension to decrease when the tire is inflated.

First of all I’d use much more tension and try to get it more even. Currently you have about 30% difference between slackest and tautest spoke, which is a lot (I’d aim for ~10%, but I’m a perfectionist in that regard).

540N on the slackest rear spoke before you’ve even inflated the tire is very little. I’d get the drive side spokes to slightly over the allowed 1200N and hope that the non-drive side ends up around 700N.

I’d also aim for 900N on the front wheel.

All tensions assuming zero tire pressure.

Considering the low spoke count and the high tire pressure you are using you could go for even higher (initial) spoke tension.

I’ve had good experience with using grease on the spoke nipples (on contact surfaces and thread). Makes truing much easier, avoids wear from truing and also avoids deforming the spokes or nipples from all the tension force.

My front and rear wheels needed truing, so I picked up tools to do it. Rims are DT Swiss R460: 20 spokes front, 24 back. Max spoke tension is 1200N.

Ouch!

1200N is a reasonable spoke tension for a 36-spoke wheel. To keep 20 spoke wheel in tension, it would need 2160 N tension, and to keep 24 spoke wheel in tension, it would need 1800 N tension.

That decrease isn't normal -- is it?

It's exactly what you would expect for a low spoke count wheel. The wheel stands on the few bottommost spokes. The fewer spokes there are, the fewer spokes the wheel stands on. A severe bump in the road and the momentary load can be 200-300 kg (2000 - 3000 N). A 20-spoke or 24-spoke wheel cannot withstand that load, so the bottommost spoke is slack and the nipple on the threads is free to turn due to vibration.

As a bad fix (not recommended), you could try some thread glue in the nipple threads. They will prevent the nipples turning freely. This is both a good thing (bump + vibration does not loosen the spokes), and a bad thing (your wheel truing just became harder).

As an ultimate fix, throw away these poor rims that were clearly designed by an incompetent engineer (who would design a rim that has both fewer than 36 spokes and maximum tension reasonable for 36 spokes?), and try to find a rim:

• That has 20 spokes and 2160 N maximum allowed tension
• That has 24 spokes and 1800 N maximum allowed tension
• That has 36 spokes and 1200 N maximum allowed tension

My recommendation is the last choice. Experience has shown that 36 spoke wheel is a reasonable compromise between low and high spoke counts and very durable.