ASRock had previously released a new beta BIOS (3.60L) for the DeskMini A300 that brought support for the Ryzen 4000-series (codename Renoir) APUs to the DeskMini A300. For reasons unknown, the motherboard vendor has since removed the BIOS from the mini PC's support page.
The fact that the system ever supported Renoir is surprising. The DeskMini A300 has an AMD motherboard with the A300 chipset, and Renoir only works on 400-and 500-series chipsets. According to Chinese publication HKEPC, ASRock's simple workaround consisted of dropping support for old Bristol Ridge chips to make room for the new Zen 2 APUs.
Being a beta BIOS, stable operation is never guaranteed. However, HKEPC found the Ryzen 7 Pro 4750G working on the motherboard without hiccups. The 3.06L BIOS is no longer available for download at ASRock's website, but if you feel like experimenting, you could probably find it somewhere on the internet at your own risk.
This could be related to recent sightings of a possible successor to the DeskMini A300. If you're not familiar with the DeskMini A300, the system employs the ASRock A300M-STX. So the sudden appearance of the ASRock X300M-STX with a Ryzen 7 4700G seemingly suggests that ASRock has already prepared a new DeskMini with an updated chipset.
Restoring support for Renoir on the current DeskMini A300 would certainly earn ASRock some brownie points. However, it might not be a wise move from a business perspective, since current DeskMini A300 owners would have no incentive to upgrade to the next best thing. But it's still possible for one to just pick up the current DeskMini A300 and flash the BIOS to get it working with Renoir.
On the other hand, if the updated DeskMini brings significant upgrades, there will be incentive to opt for the purported new model.
It remains unclear if ASRock is actually planning on releasing a DeskMini A300 replacement. Considering that AMD has already released Renoir, we'd expect to see a new DeskMini before the year ends.