Low standard land value: potential problem in property valuation?

Such jumps (25%) are rather the exception than the rule.

That may be true, but in that case the last reference values were still from the early 70s, because nothing has happened since then. The purchase price was also closer to the current reference value.

It is more likely that you have established the new value with the purchase if there is actually such a small statistical basis. But [Bodenrichtwerte] are just [Bodenrichtwerte]. The value in the end is determined by the market.

I was not the only one who bought. In total, it was about 50 plots, which of course carries a bit more weight. And unfortunately, the market does not determine the value (from the bank's perspective) but only the price. That is a difference that the OP has already sensed here.

So for 40 years there was not a single land sale for determining a standard land value and then 50 plots are sold... unusual.

You have already understood the demand and supply and price formation, right? At the same time, this then forms the corresponding market.

And regarding the lending value of a property/land, there are quite clear rules that all banks must follow. There is of course some leeway, but the bank cannot arbitrarily say anything different from what the standard land value (including explanation!) for a plot indicates. Even if it sometimes seems that way, banks do not grant wishes in this area. The legislator has a considerable say in this.

I am not at all interested in discussing price formation. I just want to point out that the price (the amount paid for the property) is not necessarily the same as the value (in terms of monetary security) from the bank's perspective, because they - as you wisely noted - have guidelines on how to determine this (sustainable) value, which are not based on the pure purchase price.