In response to bragga, apparently this is a recreation of a story, and after her transformation in the story she flashes back to how she learns to transform herself, so it's not out of order. It's just weird because there's no dialogue to indicate this.
Edit: My bad gweh, I only saw the last part of this sequence on here and assumed that's what bragga was talking about.
No, Infernnen, it's for sure out of order in a few places. See gallery pages 20-21-22 in here - and, specifically, check the filenames.
20's commission_megan_s_tail_part_02_pag_21_by_boastudio_ddxl4bg; 21's commission_megan_s_tail_part_03_pag_20_by_boastudio_ddxl40z.png, and 22's commission_megan_s_tail_part_03_pag_22_by_boastudio_ddxl4nz.png
That is to say: 20=21, 21=20, 22=22; part 2 comes first, followed by part 1, followed by part 3. (I have no clue why the above formatting's fucky, soz)
This changes the sequence into something that makes no sense; she's SUPPOSED to be expecting leg merge, getting none, frustratingly clapping her legs together, and finally triggering the merge - but instead it reads like she's imagining the legs merging, opening her eyes and realising that it hasn't happened, slapping her legs together - then POOF, tail out of nowhere.
The reason this happened is simple, mind - e-hentai orders images in alphabetical and numerical order, and the artist fucked up naming them. See above filenames; page 21 is mistakenly labeled as part of "part_02", instead of part 3 - so it gets shuffled to be right behind page 19, the ACTUAL end of part 2.
TL;DR reorder gallery pages 20-22 manually, and/or delete the images, fix filenames locally, and re-upload them so people downloading the gallery don't have to rename them themselves. This has nothing to do with weirdly-placed flashback.