Your input format is wrong: You need a decimal point, not a colon, between the seconds and the fractional seconds.
```
▶▶▶ create table foo(datetime text);
▶▶▶ insert into foo values ('2021-07-19 08:43:46.956'),('2021-07-20 10:26:41.357');
▶▶▶ select julianday(max(datetime))-julianday(min(datetime)) from foo;
julianday(max(datetime))-julianday(min(datetime))
-------------------------------------------------
1.07146297441795
```