I did a test as well, mine will properly convert up to 13 digit hex codes, after that it gets off. Definitely seems to be a limitation in powerapps. here was my code
ClearCollect(HexSplit,
ForAll(Upper(Split(TextInput1.Text,"")) As HexVals,
{Hex:HexVals.Result,
Dec:Switch(HexVals.Result,
"0", 0, "1", 1, "2", 2, "3", 3, "4", 4, "5", 5,
"6", 6, "7", 7, "8", 8, "9", 9, "A", 10,
"B", 11, "C", 12, "D", 13, "E", 14, "F", 15)}));
Set(HexFinal,
Sum(
ForAll(Sequence(CountRows(HexSplit),0,1) As stepCount,
{Formula:Text(Last(FirstN(HexSplit, CountRows(HexSplit)-stepCount.Value)).Dec)&"*16 to the "&Text(stepCount.Value),
ValMid:Sum(Last(FirstN(HexSplit, CountRows(HexSplit)-stepCount.Value)).Dec)*Power(16,stepCount.Value)}),ValMid))
even added a text box and put value(correct answer) in it and it changed it to the same value see below

what I find most perplexing about it is both the CORRECT answer and the ANSWER given are 19 numbers long, AND the given number is LARGER than the correct answer, so it's not like its hit a value cap and just stopped at the capped number.
@timl @WarrenBelz Do either of you guys see/know a way around this issue?
**Edit extra column in my set () portion of the formula was just me makign sure in a collect prior, that every step was giving right formula, ignore it