dps66678 I deleted Spinebot's answer because it was off the mark.

Unfortunately, it is difficult to judge your problem based on the screenshots you have attached. This is because the timeline for the slot attachment is only displayed when the bone to which the slot's parent is selected, so if you say that it is not displayed when you select another bone, it is possible that you are simply selecting the wrong bone. In fact, from the screenshot you showed, the bone to which the key is assigned is named bone, but the name of the bone selected when you said the key was not displayed was bone7. Also, the filter button in the dopesheet is highlighted, so it's possible that the keys are not being displayed due to the filter. There are various factors that could be causing the slot attachment timeline not to be displayed, so I cannot say which one is actually causing the problem. Could you turn off the filter and prepare a simple skeleton to show the problem of the slot attachment timeline not being displayed under as simple a set of conditions as possible?

dps66678 Thank you for explaining in detail! And I'm sorry, but when I wrote my first reply, I thought the dopesheet filter was turned on, but it seems that the Sync button is actually turned on, and I can see that this is the result of the Sync button.

When the Sync button is enabled, the dopesheet shows the rows for the curves that are visible in the graph. In the graph view, the slot attachment timeline is not displayed by default (because the slot attachment key does not have a curve). Therefore, when you select the root bone, the slot attachment time line is hidden in the graphics view, and even if you select a bone after that, the slot attachment time line remains hidden in the graphics view, because the display in the graphics view is managed by the display in the dopesheet. If you do not want this behavior, disable the Sync button.