The relevant rules are in 2019/2020 UEFA Champions League Regulations:
Article 23 Confirmation of fixtures and automatic reversals
23.01 The venues, dates (in accordance with the calendar in Annex C) and kick-off times
for all qualifying matches except those in the preliminary round are fixed by the
home clubs and must be confirmed to the UEFA administration in writing by their
respective associations by the deadline set by the UEFA administration. The UEFA
administration may alter or confirm dates and kick-off times in accordance with
the principles set by the Club Competitions Committee.
23.02 The UEFA administration decides on match dates and fixture reversals on a caseby-case basis and in accordance with the principles set by the Club Competitions
Committee. The UEFA administration reserves the right to impose a match date in
the event of clashes between matches in this competition and other UEFA club
23.03 If more than one club from the same city, or within a radius of 50km (31 miles) of each other, is taking part in any of the UEFA club competitions and/or plays in the same stadium, and if the association and the clubs concerned explicitly declare when entering the clubs that their matches cannot be played on the same day or on consecutive days, the UEFA administration may alter or confirm dates and kickoff times in accordance with the principles set by the Club Competitions Committee.
Article 24 Venues and kick-off times
24.01 The calendar and kick-off times from the play-offs onwards are established by the
24.02 The play-offs, group matches (subject to Paragraph 24.03), round of 16 matches,
quarter-finals, semi-finals and final kick off at 21.00CET. Exceptions to this rule can
be set by the UEFA administration.
24.03 Each matchday in the group stage, two of the Tuesday matches and two of the
Wednesday matches kick off at 18.55CET. Exceptions to this rule can be made by
the UEFA administration.
24.04 The fixtures within a group on the last matchday are played simultaneously
Only the games on the last matchday are synchronized, to prevent unsportsmanlike behavior and/or controversy in case if one of the teams (or both teams) in the last game know what kind of result they need to achieve (examples: Argentina - Peru (WC1978), Germany - Austria (WC1982), Denmark - Sweden (Euro2004).
We can assume that UEFA doesn't expect teams to play strategically having two games ahead.
At most two group stage matches on a given day can start earlier (at 18:55 CET). UEFA decides which games to put in this time slot. The administration takes requests from home clubs into account and tries to put geographically close teams at different times. We can suppose that timezones are also taken into consideration as it seems clubs from Eastern Europe get earlier times more often than others.
And in this particular case (Matchday 4, Wednesday) the two easternmost matches started earlier:
This isn't always the case, though. Consider Matchday 2, Tuesday:
Earlier times were given to Real Madrid and Atalanta (Bergamo) much to the chagrin of Lokomotiv fans. It's unclear how exactly UEFA arrived at a decision like this. One Russian web site published a response from the unnamed representative of UEFA press services listing a number of factors taken into account :
sports justice, fan experience, commercial preferences, and practical limitations.
Conclusion: only the games on the last matchday have to be synchronized. On a given day UEFA puts at most two matches to an earlier time slot using some obscure behind-the-scene process.