It looks like RTM is interpreting this as a due date of 1335 on the 5th of the next month. You can see that there's a due date assigned on the 5th of the next month. Since 1335 is not a valid time that RTM can interpret, it drops it as invalid. At least that's my theory.

If you type in "5335 John Street - Research X" (without quotes), it appears OK since there is nothing there that resembles a date or time.

I also noticed that if you add a single quote (or a double quote) at the beginning of '1335 5th Street - Research X, then it takes it without trying to interpret the numbers.

I hope this helps.

I also discovered that if you edit the task name after adding a task, it will take whatever you give it. So, after creating the task "1335 5th Street - Research X" (without quotes) and ending up with "Street - Research x," you can then edit the name to add back in the numbers and they will stay.

I realize it's a drag to have to use that extra step, but it is a workaround anyway. Still much faster to just start with a single quote. And if that quote at the beginning is an eyesore, or if it disrupts your sorting, you can edit the name after creating the task to remove it.