First, raising the number to 30 decimal places proves nothing (not even rounding errors).
Second, Google Sheets is not able to handle more than 15 decimals (for example, everything that follows is irrelevant)
Third, all numbers in Google Sheets also serve as surrogate parameters for dates, for example:
1 = 31/12/1899 100 = 09/04/1900 -1000 = 04/04/1897 43555 = 31/03/2019 100000 = 14/10/2173
In addition, there is a certain threshold (+/- 10000) around the date of today, which reflects in such a way that Google Sheets predicts that it is a problem. a date and not a number. So, to answer your question: these strange numbers around the 11th decimal place are due to the fact that Google Sheets converts these numbers into dates, for example. you are adding date values instead of numbers.
Therefore, to fix it, you will have to force the numbers to the date values, which will give you: