According to DigiTimes, AMD has changed the plans to introduce new architecture featuring Zen3 cores on a 7nm EUV fabrication node. The company is now rumored to launch Ryzen 4000 series on 5nm node (5nm+) instead.
The TSMC foundry has only begun mass production of 5nm wafers in April, with an enhanced 5nm+ node planned to enter production in Q4. This will allegedly affect plans for Ryzen 4000 launch, as AMD was initially targeting September/October, Digitimes reports.
UPDATE: AMD CEO Lisa Su commented on the rumor during Bernstein’s “Annual Strategic Decision Conference”. She stated that new nodes usually start with mobile processors as these are easier to manufacture. She did not mention Zen3 when talking bout 5nm node, but she did mention Zen4 a few times. Source
To me that sounds like it's mostly debunked by their own update.