R plotly formatting xaxis tickmark - how to suppress seconds?

Hi all,
I seem to spin in circles and cannot find any pointer. I want to plot a simple linechart with the xaxis time being formatted as hour:minutes, i.e. %H:M. Specifying, tickformat = “%H:%M” does not work.

Even when setting tickformat deliberately to %H only (c.f. below), I produce the following chart with the time ticks in %H:%M:S.
What do I overlook?

xax = list(title= "", type = 'time', tickformat = "%H")
yax = list(title= 'Average throughput [flights/hour]')

 thru %>% 
   plot_ly(x = ~TIME, y = ~N_BIN_AVG, color = ~SRC_PHASE) %>%
   group_by(SRC_PHASE) %>%
   add_lines(type = 'scatter') %>%    #, mode = 'markers+lines')
   layout(xaxis = xax, yaxis=yax)