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No plugins are installed; I chose the clean install (without importing settings from QGIS 2). And I've confirmed that the project is not set to use predefined scales. There are predefined scales available under Map Tools options; I have not added nor deleted any. This matches the conditions in QGIS 2.18, where the zoom function performs as expected.

I have experienced the same issue on my MacBook Pro (Early 2015 MBP w/ Retina Display, OS 10.13.3). The map view 'jumps' back to a larger scale after releasing the trackpad from zoom. I have also noticed that, whereas in previous versions of QGIS I was able to adjust the map view zoom even when e.g. the layer attributes dialog was open, if I try this now the map zooms out regardless of which direction I scroll (it is possible to zoom out, but not to zoom in).

EDIT: I found Nathan's comment about the exact doubling of the scale factor intriguing, so I decided to compare scale factors on my machine. I found that the scale factor increases by exactly 1.5x when I release the trackpad. Incidentally, my zoom factor (in the map tools preferences) is set to 150%. Upon I changing my zoom factor to 100%, the glitch no longer occurs. Nathan, I suppose that you have your zoom factor set to the default 200%?

Yes, that's right. The jump on trackpad release is according to the zoom factor; I realize I didn't make that clear. It's the actual increment of zooming that is no longer according to the zoom factor. For example, in QGIS 2.18 a single "click" of the mouse wheel—or its trackpad equivalent—has the same effect as cmd+ or cmd-, meaning it would zoom in or out by a single zoom factor. In QGIS 3.0, a single "click" moves at a much smaller increment than the zoom factor (intentionally), but it's not an even division, so I cannot "land" on a 2x multiple of the original scale. (Perhaps that's why it "jumps"—is it trying to jump to a multiple of the zoom factor, but instead jumps by a multiple of the zoom factor?)

I only have this problem (map zooming out upon completing zoom with the trackpad) in OSX when on the "Pan" tool...when on any other tool (identify, select, zoom in, zoom out, add point, move nodes are all the tools I tried), the final zoom level when I release my fingers from the trackpad seems is the same zoom level when my fingers were on the trackpad. Perhaps it is related to the Pan tool?

Yes, I can confirm that the jump in scale only occurs with the Pan tool. With other tools, however, the other part of the issue still remains (zooming by small increments not related to the scale factor).

I am also experiencing this exact same issue on macOS 10.13.5 (Early 2013 MBP w/ Retina Display, 13 inch) with QGIS 3.2.0 running Qt 5.9.3. This was installed using the installation wizard, without importing settings from QGIS 2.

Nathan Perry wrote:

Yes, I can confirm that the jump in scale only occurs with the Pan tool. With other tools, however, the other part of the issue still remains (zooming by small increments not related to the scale factor).

I confirm what Nathan said. The bug is still here in 3.2, and it’s associated with pan. Another thing is that the bug is not just trackpad related, since It happens using Apple Mouse.

I confirm this problem. I can only work with Qgis 3.2 by using an old USB mouse on my MacPro running 0SX 10.3.5. With either the Trackpad or the Magic Mouse it is unusable due to the constant zooming out: impossible to get anything done.