I've been creating some tag excerpts, but somehow they've made it into my review queue.
Of course, I can't accept or reject them, they're just there.
Is this a bug or by design?
Its actually a bug imho, you should not see suggestion you made in the queue. You can not act on them.
Resolved in dev.
For normal edits, this doesn't come up because the reputation threshold for full edit rights (2000) is the same threshold as for approving edit suggestions (2000). There is a possibility to see your own edits in the queue, but only if you can suggest one and then make it to 2000 before it gets addressed.
However, the thresholds for tag wikis differ - full edit rights come at 20000 reputation, while approval rights come at 5000. As such, you have approval capability during the period which you still must suggest edits. This is mostly intended not so much out of a lack of trust for editing privileges, but more to impose a very basic peer review system for the tag wikis.
The UI already notes that it is your own suggestion and appropriately bars you from approving or rejecting it, as you've noted by your own experience.