Apple has recently removed the Mac mini's higher memory options from the Apple Store build-to-order configuration tool with speculation that RAMageddon being the likely cause. However as of April 23, many of the Mac mini options are either listed as shipping in 10 to 12 weeks, or in the case of the non-Pro M4 chip, "unavailable".
One configuration puts the shipping date occurring after WWDC which could indicate Apple is preparing for a new M5 Mac mini, however this messaging is not consistent across the configurator. The $599 base model with 16GB of memory and 256GB of storage (not exactly a model we would recommend given the minimal RAM and SSD) is "Currently Unavailable" suggesting very likely this is not about the RAM shortage.
Since there's very little consistency between memory and storage, this seems like the base model could be expecting a refresh though there being some models still available months out is odd. Possibly these configurations were built and are currently on boats—in the supply chain making their way to Apple's distribution given Apple has been very clear that the Mac mini is their most carbon neutral Mac and they've previously explained they're preferring slow ocean-based shipping over fast but polluting air crafts.
While the base model with 256GB of storage is universally out of stock, several models with more memory or more storage are available. However the 32GB memory model is complete out of stock which could be due to RAM shortages. (64GB M4 Pro models are in stock though).