It depends how you define a mule. Rarity shouldn't come into it. In the case of currency coins, traditionally a mule is a combination of dies, one of which is either obsolete or shouldn't be used for the issue in the first place. e.g the undated 2008 20p, where clearly it was the intention to date every coin, or say the groats in the last DNW sale which had a 3d obverse paired with a 4d reverse.
So the question arises whether one or the other of the dies has clearly been superseded at an earlier date making its use anachronistic. Using this definition the answer has to be no because you have F528 to F530 using obverse 3 and F531 to F541 using obverse 5, F528 uses reverse B which was the normal one since 1860 (1874 excepted, but this say the introduction of the Heaton coins) and F529 to F540A uses reverse C. Clearly this was a changeover from one design to a slightly modified version of both obverse and reverse, but dies being used to destruction requires some overlap.
To expand. ESC lists the 1711 shilling with the third bust as 'probably a mule'. This is reasonable as there is a distinct issue of 4th bust shillings dated 1710, but the R2 rating indicates a fairly rare item, so it is not clear whether the 3rd bust die was resurrected or if it was part of the natural changeover pattern.