  The lack of short baselines, referred to as short-spacing problem (SSP), is a
well-known limitation of the performance of radio interferometers, causing a
reduction of the detected flux from large scale source structures. The very
large number of antennas operated in the Atacama Large
Millimeter/sub-millimeter Array (ALMA) generates situations for which the
impact of the SSP takes a complex form, not simply measurable by a single
number, such as the maximal recoverable scale. In particular extended antenna
configurations, complemented by a small group of closeby antennas at the centre
of the array, may result in a double-humped baseline distribution with a
significant gap between the two groups. In such cases one should adopt as
effective maximal recoverable scale that associated with the extended array and
use only the central array to recover missing flux, as one would do with single
dish or ACA (Atacama Compact Array) observations. The impact of the missing
baselines can be very important and may easily be underestimated, or even
overlooked. The present study uses ALMA archival data of the $^{29}$SiO(8-7)
line emission of AGB star W Hya as an illustration. A critical discussion of
the reliability of the observations away from the star is presented together
with comments of a broader scope. Properties of the circumstellar envelope of W
Hya within $\sim$15 au from the star, many of which are not mentioned in the
published literature, are briefly described and compared with R Dor, an AGB
star having properties very similar to W Hya.
