SUMMARY
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***


STEPS TO REPRODUCE
1. Login in a plasma6 wayland session
2. KSplash keeps loading for 30 and more seconds
3. After this delay, the desktop is shown normally

OBSERVED RESULT
If you move mouse  pointer on topleft of the screen during KSplash loading, the desktop is shown and seems up and running. If you move back the mouse, KSplash returns in fullscreen mode.
If I select the only one alternative of KSplash theme (Kuro) it works as the default one (30 seconds of delay).
If I disable KSplash desktop is shown just after login (with the same delay I had in Plasma 5)

EXPECTED RESULT
After login KSplash must be shown only for a few seconds.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version:  6.0.0
KDE Frameworks Version: 6.0.0
Qt Version: 6.2.2

ADDITIONAL INFORMATION

Laptop PC Dell Inspiron 5579

This is the output of my lspci:

00:00.0 Host bridge: Intel Corporation Xeon E3-1200 v6/7th Gen Core Processor Host Bridge/DRAM Registers (rev 08)
00:02.0 VGA compatible controller: Intel Corporation UHD Graphics 620 (rev 07)
00:04.0 Signal processing controller: Intel Corporation Xeon E3-1200 v5/E3-1500 v5/6th Gen Core Processor Thermal Subsystem (rev 08)
00:13.0 Non-VGA unclassified device: Intel Corporation Sunrise Point-LP Integrated Sensor Hub (rev 21)
00:14.0 USB controller: Intel Corporation Sunrise Point-LP USB 3.0 xHCI Controller (rev 21)
00:14.2 Signal processing controller: Intel Corporation Sunrise Point-LP Thermal subsystem (rev 21)
00:15.0 Signal processing controller: Intel Corporation Sunrise Point-LP Serial IO I2C Controller #0 (rev 21)
00:16.0 Communication controller: Intel Corporation Sunrise Point-LP CSME HECI #1 (rev 21)
00:17.0 SATA controller: Intel Corporation Sunrise Point-LP SATA Controller [AHCI mode] (rev 21)
00:1c.0 PCI bridge: Intel Corporation Sunrise Point-LP PCI Express Root Port #6 (rev f1)
00:1f.0 ISA bridge: Intel Corporation Sunrise Point LPC Controller/eSPI Controller (rev 21)
00:1f.2 Memory controller: Intel Corporation Sunrise Point-LP PMC (rev 21)
00:1f.3 Audio device: Intel Corporation Sunrise Point-LP HD Audio (rev 21)
00:1f.4 SMBus: Intel Corporation Sunrise Point-LP SMBus (rev 21)
01:00.0 Network controller: Qualcomm Atheros QCA6174 802.11ac Wireless Network Adapter (rev 32)