Sounds like they've been bitten by IEEE 794 floating point problems. JS only supports encoding numbers that are representable in 64-bit ("double precision") IEEE 794. Most JSON standards make the same assumption and define JSON number to match. (There's no lack of a standard an "encoding" standard there, it just inherits JS', which is double precision IEEE 794.) Some JSON implementations in some languages don't follow this particular bit of JSON standardization and instead try to output numbers outside of the range representable by IEEE 794, but that's arguably much more an "implementation error" than an error in the standard.
This is a most common occurrence in dealing with int64/"long" numbers towards the top or bottom of that range (given the floating point layout needs space).
There is no JSON standard for numbers outside of the range of double precision IEEE 794 floating point other than "just stringify it", even now that JS has a BigInt type that supports a much larger range. But "just stringify it" mostly works well enough.
The JSON "Number" standard is arbitrary precision decimal[1], though it does mention that implementations MAY limit the parsed value to fit within the allowable range of an IEEE 754 double-precision binary floating point value. JSON "Number"s can't encode all JS numbers, since they can't encode NANs and infinities.
The "dual" standard RFC 8259 [1] (both are normative standards under their respective bodies, ECMA and IETF) is also a useful comparison here. It's wording is a bit stronger than ECMA's, though not by much. ("Good interoperability" is its specific call out.)
It's also interesting that the proposed JSON 5 (standalone) specification [2] doesn't seem to address it at all (but does add back in the other IEEE 754 numbers that ECMA 404 and RFC 8259 exclude from JSON; +/-Infinity and +/-NaN). It both maintains that its numbers are "arbitrary precision" but also requires these few IEEE 754 features, which may be even more confusing than either ECMA 404 or RFC 8259.
One example that's bitten me is that working with large integers is fraught with peril. If you can't be sure that your integer values can be exactly represented in an IEEE 754 double precision float and you might be exchanging data with a JavaScript implementation, mysterious truncations start to happen. If you've ever seen a JSON API and wondered why some integer values are encoded as strings rather than a native JSON number, that's why.
