MantisBT - Open CASCADE
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0028672Open CASCADE[OCCT] OCCT:Data Exchangepublic2017-04-21 11:542020-09-17 19:54
[OCCT] 7.6.0* 
0028672: Valid shape is written to STEP-file as invalid (self-intersected)
See Steps To Reproduce.

The face a_1_20 really seems to be valid (see Valid face.PNG attached picture).

After write/read, we obtain the face, which is valid in terms of checkshape but is self-intersected indeed (see Invalid face.PNG attached picture).
Test de step_2 O9

stepread [locate_data_file pe_soapbox-B.stp] a *
checkshape a_1
# This shape seems to be valid ==> OK

explode a_1 f
don a_1_20
# see Valid face.PNG

stepwrite 0 a_1 d:/file.stp
stepread d:/file.stp b *

checkshape b_2
# This shape seems to be valid ==> WRONG

explode b_2 f
don b_2_20
# see Invalid face.PNG
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png Valid face.PNG (9,998) 2017-04-21 11:54
png Invalid face.PNG (13,696) 2017-04-21 11:54
Issue History
2017-04-21 11:54nbvNew Issue
2017-04-21 11:54nbvAssigned To => gka
2017-04-21 11:54nbvFile Added: Valid face.PNG
2017-04-21 11:54nbvFile Added: Invalid face.PNG
2017-04-21 11:57nbvNote Added: 0065415
2017-04-21 11:58nbvNote Added: 0065417
2017-07-27 11:15abvTarget Version7.2.0 => 7.4.0
2019-09-04 12:58abvTarget Version7.4.0 => 7.5.0
2020-09-17 19:54gkaTarget Version7.5.0 => 7.6.0*

2017-04-21 11:57   
If we try to write single face only to stp-file it will be written as valid. I.e. the face becomes invalid after write full source shape as it has been done in Steps To Reproduce.
2017-04-21 11:58   
After the fix #28211, problem face will be classified by checkshape as "Bad orientation".