Please see the bug id 211027. This bug vanished at kernel 5.10.6 and later versions of 5.10 and appeared again from kernel 5.11.x. Simplest test methodology I found is to set display standby to one minute, then turn off your HDMI display physically (or cut power to the display itself) and then wait >1 minute and turn the display back on. After a keypress or mouse movement the expected behaviour is to wake the system up again, what really happens is either a black screen or a dead picture from the last screen that was shown before the 1 minute display standby. But the system nearly completely locks. Only way for recovery is (it only works sometimes) to change to TTY2 or another TTY that is not runing the display server and "reboot". I am very sure this is coming from the kernel, because the behaviour survives driver changes from the GPU. AMD 2400 with internal radeon graphics, mesa drivers. Please check what I wrote about this bug in https://bugzilla.kernel.org/show_bug.cgi?id=211027
It is even easier to crash your linux. Just unpower your HDMI display (really unpower it) while linux is still running and power it on afterwards. I really wonder how this is not more broadly recognized.
I do have this bug also, processor : 11 vendor_id : GenuineIntel cpu family : 6 model : 166 model name : Intel(R) Core(TM) i7-10710U CPU @ 1.10GHz stepping : 0 microcode : 0xe0 cpu MHz : 756.831 cache size : 12288 KB physical id : 0 siblings : 12 core id : 5 cpu cores : 6 apicid : 11 initial apicid : 11 fpu : yes fpu_exception : yes cpuid level : 22 wp : yes flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc art arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aperfmperf pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb invpcid_single ssbd ibrs ibpb stibp ibrs_enhanced tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid mpx rdseed adx smap clflushopt intel_pt xsaveopt xsavec xgetbv1 xsaves dtherm ida arat pln pts hwp hwp_notify hwp_act_window hwp_epp md_clear flush_l1d arch_capabilities vmx flags : vnmi preemption_timer invvpid ept_x_only ept_ad ept_1gb flexpriority tsc_offset vtpr mtf vapic ept vpid unrestricted_guest ple pml ept_mode_based_exec bugs : spectre_v1 spectre_v2 spec_store_bypass swapgs itlb_multihit bogomips : 3199.92 clflush size : 64 cache_alignment : 64 address sizes : 39 bits physical, 48 bits virtual power management: No LSB modules are available. Distributor ID: Ubuntu Description: Ubuntu 20.04.2 LTS Release: 20.04 Codename: focal
*** This bug has been marked as a duplicate of bug 211027 ***