The A-side of the MAX14933 (I/OA1 and I/OA2) cannot go lower than 600mV. The B-side of the MAX14933 (I/OB1 and I/OB2) are regular open-drain digital I/Os and can go down to 0V. TOC9 – TOC12 in the datasheet show the typical behavior of I/OA1, I/OA2, I/OB1, I/OB2.
This is normal behavior of a bi-directional isolator. When driving from side A to B, there will be a delay around 750mV on the A side, which is shown in TOC9. When driving from side B to A, A side low voltage will be 600mV minimum and 900mV maximum, depending on the load current, as shown in EC table and TOC11 and TOC12. This is due to an internal control loop on the A side. Since MAX14933 is a bi-directional isolator, both sides are able to drive the signal. To prevent a latch-up condition, we have a control loop on the A side to determine which side is truly driving. This control loop introduces this delay when A side is driving (total loop delay). It also holds the voltage at about 750mV typical at A side when B side is driving. All the bi-directional isolators in the market have this behavior to solve the latch-up issue.
In most cases, this will not affect the I2C behavior, since 750mV is below the input low threshold and the glitch is very short compared to the I2C speed. The master will still consider this signal as logic low and read the data correctly.